There are many technologies available to directly harness the sun's energy, the most prevalent of which are photovoltaics and solar thermal (also known as concentrated solar power or CSP). Installed CSP plants, which use conventional heat engines to generate electric power from a temperature gradient, typically operate at 14-16% efficiency. Solar thermal technologies produce electric power from a temperature gradient, traditionally by using conventional heat engines. Solid state heat engines, in the form of thermoelectric generators (TEGs), can also exploit this temperature gradient to generate power. Solar thermoelectric generators (STEGs) are solid state heat engines that generate electricity from concentrated sunlight. STEGs can operate at higher temperatures than CSP systems and do not require moving generator parts or working fluids.
Solar thermoelectric generators (STEGs), like photovoltaic systems and concentrating solar power plants, generate electricity by harnessing the energy of sunlight. Because STEGs are solid-state devices, their lack of moving parts or need for high-temperature operating fluids and their robustness in harsh environments make them an attractive technology for standalone power conversion or in hybrid solar-thermal systems. While the traditionally low efficiency of thermoelectric devices has relegated their use to such applications as waste heat recovery, recent improvements in thermoelectric devices may make STEGs a viable technology for direct solar-to-electric energy conversion.
Advances in thermoelectric (TE) materials have brought renewed attention to the potential for STEGs to serve as part of a renewable energy portfolio. Solar thermoelectric generators rely on the Seebeck effect, which describes the tendency of free charge carriers in a temperature gradient to diffuse from the hot to the cold side. These solid state devices are attractive for the following reasons: (a) the working fluid is simply charge carriers (electrons and holes), (b) they can operate at extremely high temperatures, enabling a high Carnot efficiency, (c) they are majority carrier devices which can tolerate high levels of defects compared to photovoltaic devices, and (d) all wavelengths of sunlight can be absorbed.
The first documented STEG design dates from 1888, when Weston patented a device that concentrated solar radiation onto a thermoelectric module with a black absorber surface. See U.S. Pat. Nos. 389,124 and 389,125 issued to Weston on Sep. 4, 1888, both of which are incorporated herein by reference in their entireties. Subsequently, U.S. Pat. Nos. 527,377 and 527,379, issued to Severy on Oct. 9, 1894, described a STEG that included a pump to supply cooling water to the cold side of the TE module, a battery to store the generated electrical energy, and an adjustable tracking device. The entire disclosures of the Severy patents are incorporated herein by reference in their entireties. U.S. Pat. No. 1,077,219, issued to Coblentz on Oct. 28, 1913 (“Coblentz”), describes the first experimental results for a STEG with a hot side temperature of 100° C. The entire disclosure of Coblentz is incorporated herein by reference in its entirety.
U.S. Pat. No. 588,177 issued to Reagan on Aug. 17, 1897 (“Reagan”), describes an application of solar heat to thermo batteries. The entire disclosure of Reagan is incorporated herein by reference in its entirety.
U.S. Pat. No. 608,755 issued to Cottle on Aug. 9, 1898 (“Cottle”), describes an apparatus for storing and using solar heat. The entire disclosure of Cottle is incorporated herein by reference in its entirety.
In 1954, Maria Telkes reported the first experimental STEG efficiency using flat-plate collectors in combination with a ZnSb/BiSb thermocouple. M. Telkes, Solar Thermoelectric Generators, J. APPL. PHYS. 1, 13, 25 (1954), the entire disclosure of which is incorporated herein by reference in its entirety. This device demonstrated 0.6% efficiency, which increased to 3.4% when a 50-fold concentrating lens was added. After this initial study, experimental STEG work has been intermittent, with low efficiency values due to relatively low hot side temperatures and the lack of vacuum encapsulation to prevent convective losses.
Telkes's 1954 results were not surpassed until 2011, when Kraemer, et al. experimentally demonstrated 4.6% efficiency in a Bi2Te3 nanostructured STEG. D. Kraemer, et al., High-Performance Flat-Panel Solar Thermoelectric Generators with High Thermal Concentration, NAT. MATER. 10, 532-538 (2011), the entire disclosure of which is incorporated herein by reference in its entirety. Important features of this design included a selective absorber as a thermal concentrator and the use of a vacuum enclosure to minimize conductive and convective losses. A summary of the experimental results to date (with references) can be found in Table A of the Appendix attached to the provisional application (U.S. Provisional Patent Application Ser. No. 61/769,006).
A paper by G. Chen entitled “Theoretical Efficiency of Solar Thermoelectric Energy Generators,” modeled STEGs using a thorough the constant property model (“CPM”) approach to highlight the important design variables. J. APPL. PHYS. 109, 104908 (2011), the entire disclosure of which is incorporated herein by reference in its entirety. Chen advocates for increasing the efficiency of the STEG by using a selective absorber to maximize the net flux into the thermal concentrator. Chen predicts that STEG efficiencies of approximately 12% can be achieved with 10-fold optical concentration and 200-fold thermal concentration, for a module operating at 527° C. with an average zT of 1.
A paper by McEnaney, et al. modeled segmented and cascaded Bi2Te3/skutterudite STEGs, using data from currently existing thermoelectric materials and selective surfaces. K. McEnaney, D. Kraemer, Z. F. Ren and G. Chen, Modeling of Concentrating Solar Thermoelectric Generators, J. APPL. PHYS. 110 (2011), the entire disclosure of which is incorporated herein by reference in its entirety. The efficiency of the cascaded design was predicted to be the highest, reaching 16% at 600° C. While this study did much to shed light on the important design variables of a STEG, the predicted device performance was determined by finite element modeling for a specific generator design and TE materials. Thus to date, STEG modeling efforts have been numerical approaches or have used CPM to address TE performance, limiting the applicability of these models.